Handrails for escalators, moving walkways and the like perform an important function and can serve as a safety component of the system. The handrail is required to provide a firm grip for the passenger and yet it has to be sufficiently flexible to bend around various drive wheel mechanisms and strong enough to withstand several tens of thousands of Newtons of tensile force.
Canadian Patent 898,726 discloses a widely used type of handrail construction having a standard C-shaped cross-section with longitudinally extending stretch inhibitor, body reinforcing fabric plies and a slider member, all joined together in a molded rubber composition. The stretch inhibitor is provided as an integral band of several steel wire cables which are embedded in a rubber body matrix. The wire cables are under tension and are sufficient in number to meet the load specification of approximately 30,000 Newtons tensile strength.
The handrail is often the single most expensive component to replace in an escalator or moving walkway, and the tension under which the handrail is run is one of the main variables that affect its life. The handrail forms an endless loop around the top and end surfaces of the balustrade of the escalator or moving walkway, commonly sliding on metal or plastic guides, and returns out of sight inside the balustrade or supporting structure, generally in line with the steps. It is on this return run that the drive mechanism of the handrail is placed. Most handrail drives require that there be no excessive slack in the handrail to avoid buckling and kinking of the handrail leading to damage, while excessive tension in the handrail leads to high friction forces on the guided portions of the unit and undue distortion of the flexible handrail around pulleys and roller clusters, leading to untimely wear and fatigue failure.
All escalators and moving walkways have some means of taking up excess slack in the handrail loop, by mechanically adjusting the length of the handrail path; such adjustment correspondingly adjusts the handrail tension, and accommodates increases or decreases in handrail length over its life. However, incorrect setting of this adjustment often leads to over-tensioning of the handrail resulting in over-heating of the handrail and reduced service life. Current methods used in the industry to judge the correct tension in the handrail are subjective and can require levels of skill and experience not always available. Some of the typical known methods (and figures showing these methods are described below) are:                (i) The handrail is grasped by a technician at either of the newel ends and pulled in a normal, horizontal direction and the tension judged by distance the handrail moves.        (ii) For an escalator, at the lower curve of the balustrade the tension normally lifts the handrail from its usual position sitting on top of the handrail guide. Pressing the handrail down at the centre of this curve until it sits back onto the guide allows a skilled mechanic to judge the handrail tension. This technique cannot be used on moving walkways.        (iii) By dismantling part of the balustrade it is possible in some types of escalator to see the return run of the handrail and to judge by the amount of sag between two supporting points whether it is too tight or slack. This technique lacks precision and requires costly maintenance time.        
None of these methods accurately and repeatably measure the actual tension in the handrail, although the assignee of the present invention has found that by employing a strain gauge to press the handrail down as in method (ii) it is possible to better compare handrail tensions between similar escalators using the same type of handrails (although this concept has not previously been publicly disclosed). As noted, this method is however limited to escalators and cannot be used on moving sidewalks which are horizontal and do not have suitable curves; they are also typically much longer than escalators and therefore more often exposed to incorrect tensioning.
Most devices available for measuring tension in elongate and flexible belts, films, fabrics etc. rely on the well known tensiometer which applies a force at the centre point on one side of the article, while supporting the article, between two supports on the opposite side of it. The force required to deflect the centre point relative to the support points is used to calculate the tension in the article. Typically these products are flat or rope-like enabling a device to contact both sides of the tensioned element or article, i.e. at the required centre and support points, at the same time. In such products, the actual tension elements comprise all or a substantial part of these products; in contrast, in a handrail, the stretch inhibitors or tension elements are embedded in the body of the handrail and are not readily accessible.